In the mid-1990's, the luminous efficiencies of green and blue LEDs using InGaN as a nitride semiconductor have surpassed the level of incandescent bulbs. Since then, the application of LEDs has extended to a wide range of fields, including full-color displays. Particularly, the advent of high brightness white LEDs in 1996 has opened an era of so-called semiconductor lighting. The white LEDs were fabricated by applying fluorescent materials to InGaN blue LEDs.
Conventional methods for achieving white light emission using LEDs are broadly classified into two groups of methods. The first group of methods is associated with the use of RGB multi-chips or multi-packages for white light emission. Specifically, according to Korean Patent Publication No. 2006-88940, white light emission is achieved by mixing light beams from red (R), green (G) and blue (B) chips constituting one package. The use of the RGB multi-chip for white light emission is advantageous in that the color coordinates and correlated color temperatures of white light are very easily controllable, but has the problems that 1) the phenomenon of yellow gap deteriorates the efficiency of the green LED, 2) a complex driving circuit is required due to different temperature/current characteristics and droop characteristics between the RGB chips, and 3) a sophisticated technical solution to the problem of low color rendering index (Ra) is needed for the commercialization of the LED package, leading to an enormous increase in manufacturing cost.
The second group of methods is associated with the use of single chips to achieve white light emission. Specifically, Korean Patent Application No. 2008-111832 discloses a white LED device using single chips to achieve white light emission wherein one or more chips of the single chips are phosphor-converted LEDs (pc-LEDs) using near-UV light or a blue LED as an excitation source to emit the phosphors. The white LED device using pc-LED single chips is widely used due to the relatively simple structure of a driving circuit for white light emission and high white light efficiency compared to the white LED device using an RGB multi-chip. However, the white LED device using pc-LED single chips suffer from limited correlated color temperatures, making it difficult to emit white light with various emotions (e.g., warm-white light). Another problem of the white LED device using pc-LED single chips is a low color rendering index (Ra). Since the white LED device is designed to include phosphors in one chip, energy resorption and scattering are caused between the phosphors, inevitably leading to loss of emission energy. Further, different characteristics between the phosphors cause a large tolerance of emission characteristics in the fabrication of the chips. Moreover, when three RGB chips are used to achieve white light emission, a low color rendering index (Ra) of 70 or less is observed in the correlated color temperature range of 2,700 to 12,000 K.